Microstructure Evolution Of Mg Research Articles

Achieving High Strength and Good Ductility in As-Extruded Mg-Gd-Y-Zn Alloys by Ce Micro-Alloying.

In this study, the effect of Ce additions on microstructure evolution of Mg–7Gd–3.5Y–0.3Zn (wt %) alloys during the casting, homogenization, aging and extrusion processing are investigated, and novel mechanical properties are also obtained. The results show that Ce addition promotes the formation of long period stacking ordered (LPSO) phases in the as-cast Mg–Gd–Y–Zn–Ce alloys. A high content of Ce addition would reduce the maximum solubility of Gd and Y in the Mg matrix, which leads to the higher density of Mg12Ce phases in the as-homogenized alloys. The major second phases observed in the as-extruded alloys are micron-sized bulk LPSO phases, nano-sized stripe LPSO phases, and broken Mg12Ce and Mg5RE phases. Recrystallized grain size of the as-extruded 0.2Ce, 0.5Ce and 1.0Ce alloys can be refined to ~4.3 μm, ~1.0 μm and ~8.4 μm, respectively, which is caused by the synthesized effect of both micron phases and nano phases. The strength and ductility of as-extruded samples firstly increase and then decrease with increasing Ce content. As-extruded 0.5Ce alloy exhibits optimal mechanical properties, with ultimate strength of 365 MPa and ductility of ~15% simultaneously.

Phase field simulation of microstructures of Mg and Al alloys

ABSTRACTThe phase field method has recently emerged as a powerful and versatile tool for mesoscale simulation of microstructure evolution in Mg and Al alloys. It permits the study of the evolution of arbitrary and complex microstructures without any presumption. This article provides a review of applications of the phase-field method in Mg and Al alloys. It covers the evolution of dendrites, the equilibrium shape of some key strengthening precipitates, the effect of pre-existing precipitates and dislocations on the distribution of precipitates, and strengthening effects caused by plate-shaped precipitates that are often encountered in Mg and Al alloys. To further improve the accuracy of the phase field simulation results and to further expand the phase field method to predict mechanical properties, the phase field method needs to be integrated with other methods to establish a multi-scale approach. This integration and its application on Mg and Al alloys are also reviewed.This paper is part of a thematic issue on Light Alloys.

Computer-aided cooling curve thermal analysis and microstructural evolution of Mg–5Zn–xY cast alloys

Computer-aided cooling curve thermal analysis technique was used to investigate microstructure and phase evolutions in Mg–5Zn alloy containing 0–1.5 mass% yttrium (Y) solidified at 0.7 °C s−1 cooling rate. Optical and field emission scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction analysis were performed to characterise microstructure of the cast alloys. The observations revealed that a considerable refinement, calculated as over 74%, was occurred in the alloys with increasing Y content to 1.5 mass%. The first derivative curves revealed two peaks corresponding to α-Mg and Mg7Zn3 for the binary Mg–5Zn alloy. The results also showed the evolution of three peaks related to the α-Mg, W-phase (Mg3Zn3Y2) and I-phase (Mg3Zn6Y) in the Mg–5Zn–xY alloys. The nucleation temperature of both α-Mg and I-phase decreased slightly by increasing Y content. In addition, the solidification range and total solidification time increased by 44.4 and 50% with increasing Y content.

Microstructural evolution of Mg, Ag and Zn micro-alloyed Al–Cu–Li alloy during homogenization

The microstructural evolution of a Mg, Ag and Zn micro-alloyed Al–3.8Cu–1.28Li (mass fraction, %) alloy ingot during two-step homogenization was examined in detail by optical microscopy (OM), differential scanning calorimetry (DSC), electron probe micro-analysis (EPMA) and X-ray diffraction (XRD) methods. The results show that severe dendritic segregation exists in the as-cast ingot. There are many secondary phases, including TB (Al7Cu4Li), θ(Al2Cu), R(Al5CuLi3) and S(Al2CuMg) phases, and a small amount of (Mg+Ag+Zn)-containing and AlCuFeMn phases. The fractions of intermetallic phases decrease sharply after 2 h of second-step homogenization. By prolonging the second-step homogenization time, the TB, θ, R, S and (Mg+Ag+Zn)-containing phases completely dissolve into the matrix. The dendritic segregation is eliminated, and the homogenization kinetics can be described by a constitutive equation in exponential function. However, it seems that the AlCuFeMn phase is separated into Al7Cu2Fe and AlCuMn phases, and the size of Al7Cu2Fe phase exhibits nearly no change when the second-step homogenization time is longer than 2 h.

Microstructure evolution of Mg–11Gd–4.5Y–1Nd–1.5Zn–0.5Zr (wt%) alloy during deformation and its effect on strengthening

Microstructure and texture evolutions during tensile and compression deformation of an as-extruded Mg–11Gd–4.5Y–1Nd–1.5Zn–0.5Zr (wt%) alloy have been investigated using in-situ synchrotron radiation diffraction and subsequent microscopy. The alloy consists of 〈101¯0〉 fiber texture, {112¯0}[0001] and {112¯0}〈101¯0〉 texture components prior to deformation. The texture evolves from [0001] to 〈101¯0〉 in tension, but from 〈101¯0〉 to [0001] in compression. The evolution of texture is attributed to the activity of the tensile twinning and non-basal 〈a〉 type slip. The tendency of texture evolution depends on the favorable texture component for the activation of above deformation modes. The grain refinement, Mg5(Gd, Y, Nd) and LPSO phases, and the texture contribute to the improvement in strength.

Phase Stability in Mechanically Alloyed Mg–Ni System Studied by Experiments and Thermodynamic Calculations

In this study, microstructural evolution of Mg–Ni alloy during mechanical alloying (MA) was investigated. Also, a thermodynamic approach was utilized to predict the most stable phases formed in Mg–Ni alloy after MA. The phase composition and microstructural properties of Mg–Ni alloy were assessed by X-ray diffractometry, high-resolution field emission scanning electron microscopy and high-resolution transmission electron microscopy. The results showed that ball milling of magnesium and nickel powder mixture for 70 h yields nanostructural Mg2Ni compound with an average grain size of ~20 nm. Thermodynamic calculations revealed that in the composition ranges of 0.0 < X Mg < 0.03 (at.%) and 0.97 < X Mg < 1, there is no driving force for amorphous phase formation. In the composition range of 0.07 < X Mg < 0.93, the change of Gibbs free energy for amorphous phase formation was more negative than solid solution. While for X Mg = 0.66 (nominal composition of Mg2Ni intermetallic phase), the change of Gibbs free energy for intermetallic phase was found to be more negative than both amorphous and solid solution phases indicating that Mg2Ni intermetallic compound is the most stable phase, in agreement with the experimental observations.

Microstructure and hardness of Mg–9Li–6Al–xLa (x=0, 2, 5) alloys during solid solution treatment

The microstructure evolution of Mg–9Li–6Al–xLa (x=0, 2, 5) alloy under different solid solution parameters was investigated. The results show that, during solution treatment at 350°C, the lamellar AlLi is precipitated from α-Mg in Mg–9Li–6Al, while the MgLi2Al is dissolved into the matrix. However, during solution treatment at 450°C, the AlLi phase is wholly dissolved into matrix, while the MgLi2Al is precipitated from β-Li. The addition of La can reduce the size of α-Mg, restrain the formation of AlLi, and make the precipitated MgLi2Al from β-Li at 450°C be finer than that in Mg–9Li–6Al. With the addition of La, the decrease of the amount of AlLi and MgLi2Al leads to a descent of hardness, while the refinement, Al–La phase precipitation, and the solution of Al atoms can improve the hardness of the alloys.

Evaluation of microstructural evolution of Mg–4Al–xRE in SIMA process based on statistical approach

Strain induced melt activation (SIMA) for achieving as nearly as possible a spheroidal semisolid structure with minimum entrapped second phase is more practically/economically advantageous compared to other techniques. One prerequisite towards arriving at an effective SIMA process is the use of appropriate methodology for the evaluation/prediction of pertinent factors in such processes. In the present work, the effects of alloying element, induced strain and holding time on three responses, namely, shape factor, particle size diameter and fraction of liquid, were investigated using response surface methodology based on a central composite design. According to statistical analysis and the developed polynomial equation models, it was found that all the first order terms of the independent parameters as well as second order term of alloying element are statistically significant for all the responses. In the case of shape factor, the second order term of time and the interactive term of induced strain and alloying element have statistical significance.

Microstructural evolution in Mg–Zn alloys during solidification: An experimental and simulation study

A comprehensive microstructural evolution of Mg–1.5, 4.0 and 5.5wt% Zn alloys with respect to the solidification parameters such as thermal gradient (G), solidification velocity (V), cooling rate (GV) and solute (Zn) content were investigated in the present study. Solidification techniques such as directional solidification and wedge casting were employed in order to obtain cooling rates between 0.05 and 250K/s. Microstructural features such as secondary dendrite arm spacing (SDAS), primary dendrite arm spacing (PDAS), microsegregration along the secondary dendrites and secondary phase fractions were experimentally determined. A solidification model that incorporates solute back diffusion, secondary arm coarsening, dendrite tip undercooling and dynamically linked with accurate thermodynamic databases is used to explain the experimental results.

Microstructure Evolution of Mg&amp;ndash;4.3Zn&amp;ndash;0.7Y&amp;ndash;0.6Zr Alloy during Solution Heat Treatment

The microstructure evolution in a Mg­4.3Zn­0.7Y­0.6Zr (mass%) alloy was investigated during solution heat treatment from 300 to 500°C for 12 h. In the as-cast and solution-treated at 300°C for 12 h states, the microstructures are consisting of I-phase and i-Mg. After solution treated at 400°C for 12 h, besides I-phase and i-Mg, some long strip precipitates-which are determined as Zn2Zr phase-, also appear in the microstructure. In the solution-treated sample at 500°C for 12 h, small amounts of Zn2Zr and Mg0.97Zn0.03 phases have been found. With the increase of heat treatment temperature, the phase constituents for different states are: i-Mg + I-phase (as-cast)1⁄4 i-Mg + I-phase (300°C for 12 h)1⁄4 i-Mg + I-phase + W-phase + Zn2Zr (400°C for 12 h) 1⁄4 i-Mg + Zn2Zr + Mg0.97Zn0.03 (500°C for 12 h). [doi:10.2320/matertrans.M2013378]

Study of the formation and thermal stability of Mg2Co obtained by mechanical alloying and heat treatment

The microstructural evolution of Mg and Co in a 2:1 atomic ratio was investigated during mechanical alloying and subsequent heat treatments. Microstructural characterization was determined using X-ray diffraction and scanning electron microscopy, while thermal stability was studied by means of differential scanning calorimetry.The results show that mechanical alloying produces amorphization and promotes greater microstructural refinement. Formation of Mg2Co requires an additional heat treatment at temperatures between 679 and 705K, depending on milling time. Additionally, it was determined that the activation energy for Mg2Co crystallization decreases from 206 to 184kJ/mol when the milling time increases from 12 to 36h, respectively. Finally, a combination of the mechanical alloying process and heat treatment phase evolutions was proposed as an optimal processing route in order to obtain the Mg2Co compound.

Influences of solid solution parameters on the microstrucuture and hardness of Mg–9Li–6Al and Mg–9Li–6Al–2Y

The microstructure evolution of Mg–9Li–6Al and Mg–9Li–6Al–2Y with the variation of solid solution parameters was investigated. Results show that, in Mg–9Li–6Al, under the condition of 340°C×0.5h, the MgLi2Al phase is dissolved in the phase of β-Li, and the AlLi phase precipitates from α-Mg phase. With the holding time being prolonged to 1h, the precipitated AlLi distributes in the whole phase of α-Mg. Under the condition of 440°C×0.5h, the AlLi phase is dissolved in the matrix, and MgLi2Al phase precipitates from β-Li. The addition of 2wt.% Y in Mg–9Li–6Al can promote the precipitation of MgLi2Al phase and restrain the precipitation of AlLi phase, causing the increase of hardness of the alloy.

The evolution of the growth morphology in Mg–Al alloys depending on the cooling rate during solidification

The comprehensive microstructural evolution of Mg–3, 6 and 9wt.% Al alloys with respect to the solidification parameters such as thermal gradient (G), solidification velocity (V), cooling rate (G·V) and solute (Al) content were investigated in the present study. Various solidification techniques, including directional solidification, wedge casting, sand and graphite mould casting, gravity casting in a Cu mould and water quenching, were employed in order to obtain wide ranges of cooling rates between 0.05 and 1000Ks–1. The microstructural length scales of Mg–Al alloys, such as secondary dendrite arm spacing and primary dendrite arm spacing, were determined experimentally and compared with published models. In addition, the solidification parameters of morphological transitions such as cellular to columnar dendrite and columnar to equiaxed dendrite were also determined. Based on all the experimental data and the solidification model, a solidification map was built in order to provide guidelines for the as-cast microstructural features of Mg–Al alloys.

Transmission electron microscopy and thermodynamic studies of CaO-added AZ31 Mg alloys

We investigated the microstructural evolution of Mg–3Al–1Zn (AZ31) alloy systems, with Ca or CaO added, by carrying out microstructural characterizations in conjunction with thermodynamic calculations. A calculated phase diagram of the Mg–Ca–O ternary system showed that CaO can be dissolved in liquid Mg so as to have 12.6wt.% Ca content in the liquid Mg at 700°C. Therefore, for a 0.3wt.% CaO-added AZ31 alloy, our thermodynamic calculation predicted a similar precipitation pathway to that of a 0.3wt.% Ca-added AZ31 alloy during the solidification process. In fact, a thermodynamic analysis of the precipitation pathway assuming the Scheil model showed that the major precipitates in both alloys were Al8Mn5, CaMgSi, Laves C15 and Laves C36, in good agreement with our experimental observation. However, a microstructural characterization of the as-cast alloys using transmission electron microscopy revealed that the spatial distribution of the precipitates was significantly different in the two alloy systems; unlike in the Ca-added AZ31 alloy, the Ca-containing precipitates in the CaO-added AZ31 alloy exhibited strong agglomeration tendencies. Moreover, in an alloy solidified at a faster cooling rate, undissolved CaO particles were observed in the precipitate agglomerates that were connected to the other Ca-containing precipitates. These results suggest that an incomplete dissolution of CaO particles in the liquid results in the agglomeration of precipitates, as the undissolved CaO particles can act as local sources, supplying Ca to the liquid, and can thus act as preferential nucleation sites for the Ca-containing precipitates forming during the solidification of the alloy.

Microstructure evolution of Mg–9Gd–2Er–0.4Zr alloy during solid solution treatment

Microstructure evolution of the cast Mg-9Gd-2Er-0.4Zr alloy during solid solution treatment at temperature of 460–520 °C for 3–12 h was investigated by using optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results indicated that the grain size and the shape of second phase were obviously changed with time and/or temperature going on. At 460 °C for 3 h, the morphology of the Mg5(GdEr) phase was changed into fragmentized island morphology and the volume faction of the phase decreased. After solution solid treatment at 460 °C for 6 h, the Mg5(GdEr) phase was already completely dissolved, but some cuboid-shaped RE-rich phase precipitated. As the temperature increased, the morphology of the Mg5(GdEr) phase was transformed into the same morphology as that at 460 °C for 6 h. It was suggested that the microstructure evolution of the alloy during the solid solution treatment was concluded as follows: Mg5(GdEr) eutectic phase→Gd/Er atom diffusing into matrix→spheroidic Mg5(GdEr) phase→cuboid-shaped RE-rich phase→grain boundary immigration.

Twinning and dynamic precipitation upon hot compression of a Mg−Gd−Y−Nd−Zr alloy

The microstructure evolution in Mg–8Gd–2Y–1Nd–0.3Zn–0.6Zr alloy subjected to hot compression at 350°C and the strain rate of 0.5s−1 was systematically investigated. Twinning and dynamic precipitation are two characteristic processes during compressive deformation. Crystallographic analysis indicates that the majority of deformation twins are of the {1 0 1¯ 1}-{1 0 1¯ 2} type, possessing the misorientation of 37.5° from parent grain about the c-axis. With strain accumulation, the twin–twin intersections lead to grain fragmentation of the original grains. Extensive dynamic precipitation is observed in the alloy. The β precipitates with irregular shapes are observed to form firstly on twin boundaries. Then the formation of rectangular β′ precipitates proceeds in the Mg matrix. The morphology of the β′ precipitates differs from that of the same phase obtained by solution and ageing treatment (i.e. T6 heat treatment), but their orientation relationships with Mg matrix keep consistent.

The influence of Cr and Y on the micro structural evolution of Mg―Cr―O and Mg―Y―O thin films

The compositional influence of Cr and Y on the microstructure of Mg―Cr―O, and Mg―Y―O films synthesized by reactive magnetron sputtering has been investigated by transmission electron microscopy, X-ray diffraction and molecular dynamics simulations. A decrease in crystallinity is observed in these films as the M (Cr or Y) content is increased. It is found that M forms a solid solution with MgO for metal ratios up to ~70% and ~50% for Cr and Y respectively. Above ~70% Cr metal ratio the Mg―Cr―O films are found to be completely amorphous. The Mg―Y―O films are composed of Mg(Y)O and Y2O3 nano crystallites, up to ~50% Y metal ratio. Above this ratio, only Y2O3 nano crystallites are found. The preferential <111> MgO grain alignment is strongly affected by the increase in M content. For M metal ratios up to ~50%, there is a selective promotion of the <100> MgO grain alignments and a decline in the <111> grain alignments.

Corrosion and passivation behavior of Mg–Zn–Y–Al alloys prepared by cooling rate-controlled solidification

Highly corrosion-resistant nanocrystalline Mg–Zn–Y–Al multi-phase alloys have been prepared by consolidation of rapidly solidified (RS) ribbons. The relation between corrosion behavior and microstructure evolution of Mg–Zn–Y–Al alloys with a long period stacking ordered phase has been investigated. In order to clarify the influence of rapid solidification on the occurrence of localized corrosion such as filiform corrosion, several Mg 96.75Zn 0.75Y 2Al 0.5 (at.%) alloys with different cooling rates are fabricated by the gravity casting, copper mould injection casting and melt-spinning techniques and their corrosion behavior and microstructures are examined by the salt water immersion test, electrochemical measurements, GDOES, XRD, SEM and TEM. To clarify the effect of aluminium addition on the improvement in corrosion resistance of the alloys, several Mg 97.25− x Zn 0.75Y 2Al x alloys with different aluminium contents are fabricated by consolidating RS ribbons and the formation of corroded films on the Mg–Zn–Y–Al alloys have been investigated. Rapid solidification brings about the grain refinement and an increase in the solid solubility of zinc, yttrium and aluminium into the magnesium matrix, enhancing microstructural and electrochemical homogeneity, which in turn enhanced corrosion resistance. The addition of aluminium to magnesium can modify the structure and chemical composition of surface films and improves the resistance to local breakdown of the films.

Microstructural evolution during hydrogen sorption cycling of Mg–FeTi nanolayered composites

This paper describes the microstructural evolution of Mg–FeTi mutlilayered hydrogen storage materials during extended cycling. A 28 nm Mg–5 nm FeTi multilayer has comparable performance to a cosputtered material with an equivalent composition (Mg–10%Fe–10%Ti), which is included as a baseline case. At 200 °C, the FeTi layers act as a barrier, preventing agglomeration of Mg particles. At 300 °C, the initial structure of the multilayer is preserved up to 35 cycles, followed by fracturing of the Mg layers in the in-plane direction and progressive delamination of the FeTi layers as observed by electron microscopy. Concurrently, an increase in the Mg grain size was observed from 32 to 76 nm between cycles 35 and 300. As a result, the absorption kinetics deteriorate with cycling, although 90% of the total capacity is still absorbed within 2 min after as many as 300 cycles. The desorption kinetics, on the other hand, remain rapid and stable, and complete desorption of 4.6 wt.% H is achieved in 1.5 min at ambient desorption pressure. In addition to showing good hydrogen storage performance, multilayers are an excellent model system for studying the relation between microstructure and hydrogen absorption/desorption kinetics.

High temperature deformation and microstructural evolution of Mg–Gd–Y alloys

High temperature deformation and microstructural evolution of Mg–Gd–Y alloys